The present invention relates to a rectifying circuit of the type using FETs (Field Effect Transistors) as rectifying elements and, more particularly, to a rectifying circuit applicable to, for example, a switching regulator which is a switching type DC stabilizing power source device.
A switching regulator is extensively used today as a power source device for stabilizing a predetermined DC voltage and DC current to be applied to an electric or electronic equipment, i.e., a load. The switching regulator includes a rectifying circuit in which rectifying elements in the form of diodes or FETs are incorporated. The problem with the rectifying circuit with diodes which are non-linear elements is that the efficiency is noticeably lowered due to losses ascribable to the forward voltage drop of the diodes. This is especially true when use is made of a power source whose output voltage is low. By contrast, the rectifying circuit using FETs, or linear elements maintains the voltage drop ascribable to an output current smaller than the forward voltage drop of diodes if the FETs each has a sufficiently low ON state resistance, thereby insuring satisfactory efficiency.
However, the conventional rectifying circuit using FETs as rectifying elements has the following problems. The output of a transformer and that of a choke coil or inductor are directly applied to between the gates and the sources of the FETs, respectively. Hence, the capacitances between the gates and the sources of the FETs each is charged in positive polarity during the ON time and discharged in negative polarity during the OFF time alternately, resulting in considerable drive losses of the FETs. Furthermore, since such drive losses increase in proportion to the switching frequency of a switching element which converts a DC voltage to an AC voltage and applies it to the transformer, the efficiency is lowered in relation to the design of a switching regulator of higher frequency.